1. Field of the Invention
This invention relates to a spark plug including a chip for spark discharge on at least one of the central electrode and the ground electrode and a method of producing the spark plug.
2. Description of the Prior Art
A spark plug including a chip for spark discharge on at least one of the central electrode and the ground electrode is disclosed in Japanese patent application provisional publications Nos. 6-188062 and 11-3765. Such a prior art spark plug includes a chip for spark discharge on a base material which is at least one of the ground electrode and the central electrode by laser welding. The chip includes noble metal or an alloy including noble metal. The laser welding between the chip and the base material forms a welding portion at an interface between the chip and the base material, wherein there is a large difference in linear expansion coefficient between the chip (Ir alloy, Pt alloy, or the like) and the base material (Ni-base alloy or the like). Because the chip is fixed to the base material through the welding portion by the laser welding, the chip is fixed to the base material with a weld (melted and solidified) portion, so that the laser welding has a higher reliability of connection than the resistance welding.
However, recently, a further higher reliability is still required because the chip size chip increases and the heat load in the ending increases progressively.
In the above-mentioned Japanese patent application provisional publication No. 11-3765, a plurality of welding portions are formed so as to be arranged in the distance increasing direction from the base material to make it thicker and to reduce the difference in the linear expansion coefficient between the chip and the base material to modulate the thermal stress to the connection portion. This Japanese patent application provisional publication only discloses the outline of the welding portions and fails to disclose the sectional structure and the details.
The aim of the present invention is to provide a superior spark plug and a superior method of producing a spark plug.
According to the present invention, a first aspect of the present invention provides a spark plug comprising: a tubular housing; a central bar electrode supported by said tubular housing in said tubular housing with electrical insulation therebetween; and a ground electrode extending from one end of said tubular housing; a chip, arranged at an end surface of a base material which is at least one of said central bar electrode and said ground electrode for spark discharge through said central bar electrode and said ground electrode, said chip including a noble metal, and a weld portion between said base material and said chip including first to nth weld layers formed by materials of said chip and said base material by laser welding to fix said chip to said base material, wherein said first to nth weld layers are successively arranged from a side of said base material in order of said first to nth weld layers in a distance increasing direction from said base material which is substantially perpendicular to said end surface, each of said first to nth weld layers has at least an overlap portion with a neighbor of said first to nth weld layers, a sum of a maximum first sectional area of said first layer and second sectional areas of said second to nth weld layers at said overlap portions is 1.4 times a third sectional area of said chip, said first, second, and third sectional areas are along said end surface, and n is a natural number more than one.
According to the present invention, a second aspect of the present invention provides a spark plug based on the first aspect, wherein said mth weld layer has a maximum fourth sectional area along said end surface which is greater than said second sectional area of said mth weld layer at said overlap portion between said mth and (mxe2x88x921)th weld layer, 2xe2x89xa6mxe2x89xa6n, and m is a natural number.
According to the present invention, a third aspect of the present invention provides a spark plug based on the first aspect, wherein said chip includes Ir of more than 50% by weight.
According to the present invention, a fourth aspect of the present invention provides a spark plug comprising: a tubular housing; a central bar electrode supported by said tubular housing in said tubular housing with electrical insulation therebetween; a ground electrode extending from one end of said tubular housing; a stress releasing layer, arranged on an end surface of a base material which is at least one of said central bar electrode and said ground electrode; a chip, being arranged on said stress releasing layer and including a noble metal, for spark discharge through said central bar electrode and said ground electrode; and a weld portion formed between said base material and said chip with materials of said base material, said stress releasing layer, and said chip by laser welding to fix said chip to said base material, wherein a linear expansion coefficient of said stress releasing layer is between those of said base material and said chip.
According to the present invention, a fifth aspect of the present invention provides a spark plug based on the fourth aspect, wherein a thickness t of said stress releasing layer is equal to or greater than 0.2 mm and equal to or smaller than 0.6 mm and xcex1 greater than (1.4xe2x88x92t)/2 where xcex1 is a ratio of a maximum sectional area of said weld portion along said end surface to a sectional area of said chip along said end surface.
According to the present invention, a sixth aspect of the present invention provides a spark plug based on the fourth aspect, wherein said chip includes Ir of more than 50% by weight.
According to the present invention, a seventh aspect of the present invention provides a method of producing a spark plug including a tubular housing, a central bar electrode supported by said tubular housing in said tubular housing with electrical insulation therebetween, and a ground electrode extending from one end of said tubular housing, comprising the steps of: providing a stress releasing layer on a side of said one end of said tubular housing on an end surface of a base material which is at least one of said central bar electrode and said ground electrode; providing a chip including a noble metal for spark discharge on said stress releasing layer and having a linear expansion coefficient between those of said base material and said chip; and fixing said chip to said base material by forming a weld layer at an interface portion of said base material, said stress releasing layer, and said chip.
According to the present invention, an eighth aspect of the present invention provides a spark plug based on said fourth aspect, wherein said weld portion includes first and second ring shape layers, said first ring shape layer is arranged between a portion of the end surface 31 of said base material and said stress releasing layer to fix said stress releasing layer to said base material, said second ring shape layer is arranged between said chip and said stress releasing layer to fix said chip to said stress releasing layer.
According to the present invention, a ninth aspect of the present invention provides a spark plug comprising tubular housing; a central bar electrode supported by said tubular housing in said tubular housing with electrical insulation therebetween; a ground electrode extending from one end of said tubular housing, at least one of said central bar electrode and said ground electrode servicing as a base material; a weld portion on said base material; and a chip on said weld portion, including a noble metal for spark discharge through said central bar electrode and said ground electrode; wherein a linear expansion coefficient of said weld portion is between those of said base material and said chip.
According to the present invention, a tenth aspect of the present invention provides a spark plug based on the fourth aspect, wherein said weld portion is arranged around said stress releasing layer.